The present invention relates to a multimedia network system for transmitting real-time communication type information such as television video signals, and storage type information such as computer files using at least one transmission path.
In recent years, optical fiber networks have been set up in trunk communication networks, satellite communications have been put into practical applications, and local area networks (LANs) have increasingly been widespread. In order to prevent a communication content from being leaked to a third party other than a party concerned in communications, it is important to constitute a network system which can attain an encryption/privacy function.
So-called information service industries that charge depending upon contents and amounts of information services via such a communication network are growing larger. For this reason, it is also important to simultaneously record and acquire charge information for information services in addition to the encryption/privacy function.
As a conventional information encryption/privacy function system, secret-key cryptosystem and public-key cryptosystem are known.
For further details of these systems, see D. W. Davis, W. L. Pric, "Network Security", edited and translated by Tadahiro Uezono and published by Nikkei McGraw-Hill Co. (1986).
These systems will be briefly described below (for further details, see the above reference).
In the secret-key cryptosystem, transmission and reception terminals share an encryption/decryption key, so that information is encrypted/transmitted and decrypted using this key.
As an encryption system of this type, a large number of systems, e.g., a simple sum encryption/permutation encryption system, an encryption system called "DES" (data encryption standard) which is standardized by U.S. Department of Commerce (National Bureau of Standard), and the like are known.
In the secret-key system, an encryption/decryption key must be determined in advance between transmission and reception terminals, and must be kept secret from a third party. If a third party knows this key, he or she can easily decrypt encrypted information.
In contrast to this, in the public-key cryptosystem, a pair of different encryption and decryption keys are used, and an encryption key is disclosed to all the terminals.
Each terminal has its own encryption key. A transmission terminal selects a key of a destination terminal from the disclosed encryption keys of the respective terminals, and encrypts and transmits information using the selected key. On the other hand, a reception terminal decrypts the received information using a decryption key paired with the selected key.
In this system, since a pair of keys with which a decryption key is difficult to determine from an encryption key are used, even when the encryption key is disclosed, a third party substantially cannot decrypt encrypted information.
Since a secret-key need not be determined in advance between transmission and reception terminals, a key will not be known by a third party when a key is determined in advance between transmission and reception terminals.
However, the above prior art systems suffer from the following drawbacks.
In the secret-key cryptosystem,
(1) an encryption/decryption key must be determined in advance between transmission and reception terminals, and a key may be known to a third party during a communication for determining the encryption/decryption key;
(2) even when the same key is repetitively used to avoid such undesirable disclosure of the key, a third party may find out the key upon comparison of a plurality of pieces of information encrypted by the same key; and
(3) if complex encryption such as the DES is performed, the above-mentioned possibility can be reduced. However, it is difficult to encrypt data having a large data transfer volume per unit time such as a digital video signal.
In the public-key cryptosystem,
(4) it is generally difficult to perform high-speed encryption/decryption processing.
Since the two encryption systems suffer from the above-mentioned drawbacks (1) to (4), it is conventionally difficult to safely encrypt real-time communication type information such as a video signal which must be transmitted at high speed.